X-linked recessive disorder characterized by mental retardation and large head, jaw, ears, and testes; premutation alleles in unaffected carriers give rise to significantly amplified repeats in affected progeny.

Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by mild-to-moderate intellectual disability. The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled...